The average velocity is the rate of change of the position of an object with respect to a change in time. On the other hand, the average speed for a given time interval is equal to the rate of the total distance traveled divided by the time interval in which that distance was traveled.
For example, suppose that a particle travels in a circular trajectory with a circumerence of 120km in one hour. Since it completes a circumference, the particle ends at the same point that it started. Then, the change in position is 0 at the end of the time interval of 1 hour, so the average velocity is 0.
Additionally, since the particle traveled a distance of 120km during that hour, then the average speed is 120km/h.